The present invention relates to an electrophotographic type image forming apparatus and a color superimposition adjustment method of an image forming apparatus, and more specifically relates to an image forming apparatus capable of automatically correcting color misregistration of a multi-color image which is caused when forming the multi-color image by superimposing color component images formed on image carriers or a transfer carrier, and also relates to a color superimposition adjustment method of an image forming apparatus, for automatically correcting color misregistration of a multi-color image.
In a conventional image forming apparatus such as a digital color copying machine, inputted image data is decomposed into respective color components to perform image processing, and then the respective color component images are superimposed to form a multi-color image. In such an image forming apparatus, however, when the respective color component images are not accurately superimposed, color misregistration occurs in a multi-color image to be formed. Consequently, there is a possibility of a decrease in the image quality.
Besides, conventionally, there is known a tandem type image forming apparatus which comprises one image forming section for each color component so as to improve the formation speed of a multi-color image. In this tandem type image forming apparatus, respective color component images are formed in respective image forming sections, and then the respective color component images are superimposed sequentially to form a multi-color image. In such an image forming apparatus, since the rotation behaviors of the photosensitive bodies of the respective image forming sections differ from each other, there tend to be differences in the transfer positions of the respective color component images. Thus, color misregistration of a multi-color image is a serious problem for tandem type image forming apparatuses.
In order to accurately superimpose the respective color component images, an image forming apparatus performs a color superimposition adjustment for correcting color misregistration of a multi-color image, and thereby forming a satisfactory multi-color image without color misregistration. This color superimposition adjustment is usually carried out by using an optical detector to detect a displacement of the image forming position of other color component with respect to the image forming position of a reference color component. Based on the detection result of the detector, a correction amount for the displacement is determined. Moreover, according to this correction amount, timings of forming respective color component images are adjusted so that the transfer positions of the respective color component images coincide with each other. In general, this correction amount is determined by transferring the respective color component images at the same timing and detecting the distance between the transfer positions of the respective color components, or by measuring the density of the multi-color image formed by superimposing the respective color components.
For example, in an image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 10-213940 (1998), the distance between the transfer positions of the respective color component images is detected, and a correction is made based on the detected amount of displacement of the transfer position. Specifically, by detecting the distance between an image formed by a reference color component and an image formed by other color component with a detector and then determining the amount of displacement of the transfer position of the respective color component images based on the detected distance, color misregistration of the multi-color image is corrected.
Further, Japanese Patent Application Laid-Open No. 2000-81744 discloses an image forming apparatus which corrects color misregistration by measuring the density of a multi-color image formed by superimposing respective color component images. More specifically, the correction of color misregistration is made so that the measured density of the multi-color image is equal to a density which is obtained when the respective color component images are accurately superimposed.
Moreover, this image forming apparatus of Japanese Application Laid-Open No. 2000-81744 repeatedly forms a plurality of same images for each color component image so as to improve the accuracy of correcting color misregistration. Specifically, according to this application, a plurality of line images are formed as the same images for each color component, and the densities of multi-color line images are detected with a detector to find the superimposed state of the respective color component line images. Then, a state in which the density of a multi-color line image detected with the detector is within a predetermined density range is considered as a state in which the respective color component line images are accurately superimposed, and a correction is made so that image formation is performed in this superimposed state, thereby correcting color misregistration of the multi-color image.
However, in the image forming apparatus of Japanese Patent Application Laid-Open No. 10-213940 (1998), since the displacement of the transfer position of the respective images is found using the detector for detecting the transfer positions of the respective color component images, there is a problem that a detector with high detection accuracy needs to be used to detect a minute displacement of the transfer position. Moreover, there is a problem that an accurate correction amount for color misregistration can not be determined due to the influence of irregularity in image formation caused by the rotational irregularity of an image carrier for forming an image to be detected, or by the rotational irregularity of a transfer carrier driving roller for driving a transfer carrier.
On the other hand, in the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 2000-81744, since the values of densities detected at a plurality of positions by performing sampling in a fixed cycle are averaged, this apparatus is relatively less susceptible to the influence of irregularity in image formation caused by the rotational irregularity of the image carrier, or the rotational irregularity of the transfer carrier driving roller for driving the transfer carrier.
However, depending on some image forming method or detection method, this image forming apparatus suffers from a problem that an accurate correction amount for color misregistration can not be determined due to the influence of irregularity in image formation. More specifically, suppose that an image formed by superimposing respective color component line images is a combined image. If the formation region of this combined image in a sub-scanning direction is short and one color component line image is formed in a region where the rotational velocity is higher or a region where the rotational velocity is lower, an accurate correction amount for color misregistration can not be determined. Moreover, if the sampling cycle is long and the number of samples is small, an accurate correction amount for color misregistration can not be determined.